Class Eustigmatophyceae (“true eyespot”) was separated from the
yellow-green Tribophyceae (Xanthophyceae) when the eyespot was found to lie
outside the chloroplast (Hibberd and Leedale 1970, 1971, 1972).Other structural differences included their
biflagellate condition although several have only a single emergent
flagellum.

The eyespot is described as “a large conspicuous orange-red body
in the extreme anterior end of the motile cell."